The overall objective of this proposal is to examine the feasibility of targeting disseminated metastatic cancer using intact antibody and antibody fragments radiolabeled with the alpha-particle emitter Bi-213 (T1/2= 45.6 min). We propose to test the following hypotheses: 1. Due to the permeable vasculature of early tumor metastases, the 45.6 minute half-life of Bi-213 is sufficiently long to target both vascularized and pre- vascularized early cancer metastases. 2. The 45.6 minute half-life of Bi-213 relaxes the constraint that the antibody exhibit no cross-reactivity with normal organs because penetration of the antibody into intact normal organs will occur after the radionuclide has decayed and also because the short 4-5 cell diameter range of alpha-particles will irradiate targeted tumor cells with minimal adjacent normal cell irradiation. The hypotheses will be tested using Neu-N transgenic mice bearing lung, liver and osteolytic bone metastases. SPECIFIC AIMS 1. Determine the relative expression of the rat/neu (analog to HER2/neu) receptor in tumors and in selected normal organs of the transgenic Neu-N murine model of breast carcinoma. 2. Determine the kinetics, in vivo, and spatial distribution, ex vivo, in tumors and normal organs of anti-neu IgG, F(ab')2, and Fab. 3. Evaluate the MTD and determine the dose-limiting organ in Neu-N mice treated with 213Bi-labeled anti-neu constructs. 4. Evaluate the efficacy of 213Bi-labeled anti-neu constructs against sub-cutaneous tumor and against lung, liver and bone metastases. 5. Perform microdosimetry to derive dose-response relationships to understand observed responses and toxicity in terms of absorbed dose. Current cancer treatment is rarely effective once the tumor has metastasized to distant sites. The eradication of such metastases requires a systemic, targeted therapy that is minimally susceptible to chemo- or radio-resistance, that is potent enough to sterilize individual tumor cells and cell clusters and that exhibits an acceptable toxicity. The work described in this application is intended to develop and evaluate such an approach.